1. Field of the Invention
This invention relates to an image pickup apparatus having an automatic focusing device and adapted for an interchangeable lens system arranged to permit use of interchangeable lenses.
2. Description of Related Art
Some of video apparatuses such as video cameras have been arranged to use an automatic focusing device operated by a so-called hill climbing method. According to the hill climbing method, a high-frequency component is extracted from a video signal obtained by an image sensor such as a CCD or the like. The focus of a photo-taking lens is adjusted by driving the photo-taking lens in such a way as to cause the extracted high-frequency component to become maximum.
Such an automatic focusing method permits accurate focus adjustment, irrespective as to whether the object is located at a far distance or at a near distance, without necessitating use of any special optical member for focus adjustment. FIG. 1 shows by way of example a case where such an automatic focusing method is applied to a lens interchangeable type video camera.
Referring to FIG. 1, a focusing lens 501 is used for focusing. A lens driving motor 511 is arranged to move the focusing lens 501 along an optical axis for performing focus adjustment. The light of a picked-up image passing through the focusing lens 501 is imaged on a light receiving plane of an image sensor 502 to be photo-electrically converted into an electrical signal, which is then outputted as a video signal. The video signal thus obtained is supplied to a CDS/AGC (correlated double sampling/automatic gain control) circuit 503. At the CDS/AGC circuit 503, the video signal is sampled and held and, then, amplified up to a predetermined level. After that, the amplified video signal is supplied to an A/D converter 504 to be converted into digital video data. The digital video data is inputted to a camera signal processing circuit (not shown) to be converted into the form of a standard TV signal. The output of the A/D converter 504 is supplied also to a BPF (band-pass filter) 505.
The BPF 505 extracts a high-frequency component of the video signal. The output of the BPF 505 is supplied to a gate circuit 506. The gate circuit 506 is arranged to extract from the high-frequency component only a signal which corresponds to a part set as a focusing-state detecting area within an image plane. The output of the gate circuit 506 is supplied to a peak hold circuit 507 to be peak-held at intervals synchronized with a period which is an integer times as much as the period of a vertical synchronizing signal. An AF (automatic focusing) evaluation value (signal) is formed by the peak holding action. Information on the AF evaluation value is taken in a microcomputer 508 disposed within a camera body and is sent out from the camera-body microcomputer 508 to an AF microcomputer 509 which is disposed within a lens unit.
The AF microcomputer 509 of the lens unit is arranged to decide a focusing speed according to the degree of focusing and a motor driving direction in such a way as to increase the AF evaluation value. The lens unit AF microcomputer 509 then causes a motor driver 510 to drive a motor 511 according to the speed and direction thus decided. The motor 511 then moves the focusing lens 501 along the optical axis to perform focus adjustment accordingly.
In the above-stated conventional image pickup apparatus, the AF microcomputer 509 is disposed on the side of the lens unit. The location of the AF microcomputer 509, however, has presented such a problem that the lens unit AF microcomputer 509 can not change the characteristics of the BPF 505 for performing focus adjustment in such a way as to enable the lens unit AF microcomputer to obtain an optimum AF evaluation value according to the state of the object and the shooting conditions. More specifically, in the case of a focus detecting method whereby information on a focusing state, i.e., a focus signal, is to be extracted from a video signal, the characteristics of the BPF 505 which is disposed on the side of the camera body must be changed according to the current shooting conditions, because the focus signal tends to be affected by changes taking place in the scene and circumstances of the object.
This invention is directed to the solution of the above-stated problem of the prior art. It is, therefore, a first object of the invention to provide an image pickup apparatus having an automatic focusing function by which an in-focus state can be stably attained for a main object of any kind under any shooting conditions.
It is a second object of the invention to arrange an interchangeable lens system to be capable of adequately performing automatic focusing.
To attain the above objects, an image pickup apparatus arranged as a preferred embodiment of the invention comprises a lens unit including a focusing lens, an image sensor arranged to photo-electrically convert a picked-up image light from an object passing through the focusing lens and to output a picked-up image signal, extracting means for extracting a focus signal corresponding to a degree of focusing from a predetermined area within a picked-up image plane obtained from the picked-up image signal, control means for forming a control signal for driving the focusing lens to an in-focus point on the basis of the focus signal extracted, driving means for driving the focusing lens according to the control signal outputted from the control means, and varying means for varying the focus signal to be extracted by the extracting means, wherein the control means and the driving means are disposed at the lens unit, and wherein the varying means is disposed at a body of the image pickup apparatus.